1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device including a plurality of circuit blocks or cells that need to have exactly the same characteristics.
2. Description of Related Art
Some semiconductor devices include a plurality of circuit blocks that need to have exactly the same characteristics. An example of such circuit blocks is an input first-stage circuit. Input first-stage circuits are connected to data pads to which external data is supplied. The timing of change of internal data generated by input first-stage circuits depends on the characteristics of the input first-stage circuits. To exactly match the timing of change of a plurality of internal data, a plurality of input first-stage circuits need to have exactly the same characteristics.
The characteristics of the circuit blocks can be made the same by arranging the circuit blocks in the same layout. Specifically, the circuit blocks may be configured so that a plurality of transistors constituting the circuit blocks have the same shapes and sizes. If such circuit blocks are input first-stage circuits, a plurality of data pads related to the plurality of input first-stage circuits may be arranged in a row so that the input first-stage circuits having the same layout are arranged along the data pads. This makes the characteristics of the input first-stage circuits almost the same. If data pads are too many to arrange in a row, the data pads may be arranged in two rows and input first-stage circuits having the same layout may be arranged along the respective corresponding data pads. Here, the input first-stage circuits assigned to one of the rows of data pads and the input first-stage circuits assigned to the other row of data pads can be symmetrically arranged to make the wiring loads in the respective input first-stage circuits the same.
When a plurality of input first-stage circuits are symmetrically arranged, the input first-stage circuits arranged in one row and the input first-stage circuits arranged in the other row are oriented in 180° different directions. In other words, a plurality of transistors included in the input first-stage circuits arranged in one row and a plurality of transistors included in the input first-stage circuits arranged in the other row have the same shapes and sizes but are laid out in 180° different directions. Consequently, currents flowing in the input first-stage circuits are in 180° different directions. If source-side resistances and drain-side resistances are different due to process conditions, then such input first-stage circuits produce a slight difference in characteristic.
FIG. 16 of Japanese Patent Application Laid-Open No. 2009-86880 discloses matching the directions of currents in circuit blocks. In this case, however, the circuit blocks have different wiring loads. Such a difference is not limited no input first-stage circuits but is common to a plurality of circuit blocks that need to have exactly the same characteristics.